1. Field of the Invention
The present invention relates to a lapping apparatus holding grains over the surface of a faceplate, namely a grinding surface. In particular, the present invention relates to an embedding tool designed to embed grains into the faceplate in the lapping apparatus.
2. Description of the Prior Art
A lapping apparatus utilizing fixed grains is well known. The grains are embedded in the surface of a faceplate, namely a grinding surface, prior to lapping process in the lapping apparatus. A ring member harder than the faceplate is employed to realize the embedment of the grains. An annular flat surface is defined on the ring member. The annular flat surface is designed to have the diameter equal to the radius of the faceplate or grinding surface, for example. The flat surface of the ring member is urged against the grinding surface. Grains are then applied to the grinding surface. When a relative movement is induced between the ring member and the grinding surface, the ring member functions to embed the grains into the grinding surface.
In general, the faceplate includes a disk made of tin defining the grinding surface, and an iron support member backing the disk. Tin and iron have different coefficients of thermal expansion, so that a slight temperature change such as a change of one or two degrees Celsius induces undulation over the grinding surface of the faceplate. The undulation has amplitude in the order of several to several hundred micrometers, for example. The undulation of the grinding surface causes a space or gap partially between the flat surface of the ring member and the grinding surface of the faceplate. The embedment of minute grains cannot reliably be achieved in the gap. The grinding surface thus suffers from uneven distribution of the grains. If the resulting faceplate is employed in the lapping process, the quantity or degree of progress of the grinding cannot be controlled at a higher accuracy.